This invention relates to a steering control valve of the type associated with a hydrostatic vehicle steering system.
A steering valve of the foregoing type is disclosed in a publication of the assignee of the present application, entitled "ZF-ROSS SERVOSTAT HYDROSTATIC STEERING, TYPE SERIES 8450 to 8455", of December 1975. Such hydrostatic steering systems are useful for relatively slow moving vehicles such as forklifts, tractors and combines to avoid the problems associated with mechanical steering connections. Hydrostatic steering systems are also useful for navigation of ships. Associated with the prior art steering control valve in such systems is a manually operable pump which acts to meter the flow of fluid from a high pressure pump during normal operation, the pump also operating as the pressure source in the event of high pressure pump failure.
To facilitate steering operation, the slide valve element associated with the steering control valve must be slidably mounted within the valve body with the least possible friction. Frictional resistance can only be minimized if radial forces pressing the slide valve element against the slide bearing surfaces of the valve body are avoided. Radial forces not only decrease any radial clearance between the valve element and the valve body, but also creates additional frictional pressure because of the prevailing fluid pressure differential between the various valve passages of the steering control valve. Thus, direct contact may occur between the valve element and the valve body to displace operating fluid and thereby result in high frictional loading. Angular displacement of the hand steering wheel from a rest position is therefore impeded by a high static frictional resistance. Such jamming of the valve element in the rest condition can only be overcome by a relatively high turning force applied to the hand steering wheel which must be abruptly reduced because of the transition to a lower sliding frictional resistance during continued turning of the hand steering wheel. Operation of the steering control valve is therefore characterized by uneven steering and poor sensing of road conditions.
The undesirable radial forces hereinbefore referred to are exerted on the slide valve element of prior art steering control valves because of two reasons. First, the driving torque applied to the steering spindle is transmitted to one end of a cylindrical torsion rod pinned to the spindle at one end of a bore formed in a drive shaft extension of the spindle. Because of manufacturing imperfections and clearances in the pinned connection, there is some misalignment between the axes of the torsion rod and the drive shaft. Axial loading will therefore bend the torsion rod causing it to exert a radial force on the valve element. The other end of the torsion rod is pinned to one end of a universal shaft at the same axial location at which the universal shaft is in driving engagement with the slide valve element in accordance with prior art arrangements. Since the drive shaft axis is always at some angle to the axis of the universal shaft, additional radial forces are exerted on the valve element as the other driven end of the universal shaft rotates eccentrically with the pump rotor to which it is drivingly coupled. The torque loading of the drive shaft and universal shaft are so angularly related as to establish a lateral reaction torque component which has the effect of tilting the slide valve element about an axis transverse to its longitudinal axis causing opposite ends of the valve element to engage the valve body within the bore slidably mounting the valve element.
It is therefore an important object of the present invention to provide a steering control valve for hydrostatic vehicle steering systems wherein undesirable radial forces on the slide valve element are substantially eliminated in the rest position of the hand steering wheel and minimized during transmission of driving torque.